Lidocaine action on Na+ currents in ventricular myocytes from the epicardial border zone of the infarcted heart

Canine Model of Ischemia-Induced Ventricular Tachycardia

INTRODUCTION

Despite advances in antiarrhythmia therapies, ventricular tachycardia (VT) is a leading cause of sudden cardiac death. Investigation into the characteristics and new treatments for this arrhythmia is required to improve outcomes and a reproducible model of VT would be useful in these endeavors. We therefore created a canine model of ischemia-induced VT.

MATERIALS AND METHODS

A pacing lead was implanted in the right ventricle in canines (n = 13) and the left anterior descending artery was occluded in two locations for 2 h and subsequently released to create an ischemia-reperfusion injury. In the 10 dogs that survived the first 48 h following the initial study, a terminal study was conducted 4-7 d later and VT was induced using premature stimulation or burst pacing through the right ventricle lead. The arrhythmia was terminated using either antitachycardia pacing or a defibrillatory shock. Multiple inductions into sustained VT were attempted.

RESULTS

Sustained VT was induced in eight of 10 dogs with an average cycle length of 335 ± 70 bpm. Multiple episodes of VT were induced. Episodes of VT exhibited different electrocardiogram morphologies and cycle lengths in individual animals.

CONCLUSIONS

This canine model provides a consistent technique for inducing multiple episodes of sustained VT. It may be useful for investigating VT mechanisms and testing novel therapeutics and treatments for patients with VT.

Cardiac Alternans: From Bedside to Bench and Back

Cardiac alternans arises from dynamical instabilities in the electrical and calcium cycling systems of the heart, and often precedes ventricular arrhythmias and sudden cardiac death. In this review, we integrate clinical observations with theory and experiment to paint a holistic portrait of cardiac alternans: the underlying mechanisms, arrhythmic manifestations and electrocardiographic signatures. We first summarize the cellular and tissue mechanisms of alternans that have been demonstrated both theoretically and experimentally, including 3 voltage-driven and 2 calcium-driven alternans mechanisms. Based on experimental and simulation results, we describe their relevance to mechanisms of arrhythmogenesis under different disease conditions, and their link to electrocardiographic characteristics of alternans observed in patients. Our major conclusion is that alternans is not only a predictor, but also a causal mechanism of potentially lethal ventricular and atrial arrhythmias across the full spectrum of arrhythmia mechanisms that culminate in functional reentry, although less important for anatomic reentry and focal arrhythmias.

The bradycardic agent ivabradine decreases conduction velocity in the AV node and in the ventricles in-vivo

Ivabradine blocks hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels, thereby lowering the heart rate, an action that is used clinically for the treatment of heart failure and angina pectoris. We and others have shown previously that ivabradine, in addition to its HCN channel blocking activity, also inhibits voltage-gated Na channels in vitro at concentrations that may be clinically relevant. Such action may reduce conduction velocity in cardiac atria and ventricles. Here, we explore the effect of administration of ivabradine on parameters of ventricular conduction and repolarization in the surface ECG of anesthetized mice. We found that 5 min after i.p. administration of 10 mg/kg ivabradine spontaneous heart rate had declined by ~13%, which is within the range observed in human clinical studies. At the same time a significant increase in QRS duration by ~18% was observed, suggesting a reduction in ventricular conduction velocity. During transesophageal pacing at heart rates between 100 and 220 beats/min there was no obvious rate-dependence of ivabradine-induced QRS prolongation. On the other hand, ivabradine produced substantial rate-dependent slowing of AV nodal conduction. We conclude that ivabradine prolongs conduction in the AV-node and in the ventricles in vivo.

Different electrophysiological effects of the levo- and dextro-rotatory isomers of mexiletine in isolated rabbit cardiac muscle

Racemic mexiletine is a widely used antiarrhythmic agent that blocks sodium channels. The effects of R-(-) and S-(+) mexiletine stereoisomers on maximum rate of depolarization ([Formula: see text]), conduction time, and repolarization have not yet been investigated in isolated cardiac preparations. We studied the effect of the R-(-) and S-(+) mexiletine on rabbit cardiac action potential parameters by using the conventional microelectrode technique. Both enantiomers at 20 μmol/L of therapeutically and experimentally relevant concentration, significantly depressed the [Formula: see text] at fast heart rates (BCLs 300-700 ms). R-(-) mexiletine has more potent inhibitory effect than S-(+) mexiletine. Both R-(-) and S-(+) mexiletine significantly inhibited the [Formula: see text] of early extrasystoles measured at 70 ms diastolic interval induced by S1-S2 stimuli. R-(-) mexiletine has more pronounced inhibitory effect than S-(+) mexiletine. Both R-(-) and S-(+) mexiletine increased significantly the ERP/APD₉₀ ratio. The time constant (τ) of recovery of [Formula: see text] was found to be τ = 376.0 ± 77.8 ms for R-(-) mexiletine and τ = 227.1 ± 23.4 ms for S-(+) mexiletine, which indicates a slower offset kinetics for R-(-) mexiletine from sodium channels than that of the S-(+) enantiomer. These data suggest that R-(-) mexiletine might be a more potent antiarrhythmic agent than S-(+) mexiletine.

Cardiac dynamics: Alternans and arrhythmogenesis

Pre-existing heterogeneities present in cardiac tissue are essential for maintaining the normal electrical and mechanical functions of the heart. Exacerbation of such heterogeneities or the emergence of dynamic factors can produce repolarization alternans, which are beat-to-beat alternations in the action potential time course. Traditionally, this was explained by restitution, but additional factors, such as cardiac memory, calcium handling dynamics, refractory period restitution, and mechano-electric feedback, are increasingly recognized as the underlying causes. The aim of this article is to review the mechanisms that generate cardiac repolarization alternans and convert spatially concordant alternans to the more arrhythmogenic spatially discordant alternans. This is followed by a discussion on how alternans generate arrhythmias in a number of clinical scenarios, and concluded by an outline of future therapeutic targets for anti-arrhythmic therapy.

Deranged sodium to sudden death

In February 2014, a group of scientists convened as part of the University of California Davis Cardiovascular Symposium to bring together experimental and mathematical modelling perspectives and discuss points of consensus and controversy on the topic of sodium in the heart. This paper summarizes the topics of presentation and discussion from the symposium, with a focus on the role of aberrant sodium channels and abnormal sodium homeostasis in cardiac arrhythmias and pharmacotherapy from the subcellular scale to the whole heart. Two following papers focus on Na(+) channel structure, function and regulation, and Na(+)/Ca(2+) exchange and Na(+)/K(+) ATPase. The UC Davis Cardiovascular Symposium is a biannual event that aims to bring together leading experts in subfields of cardiovascular biomedicine to focus on topics of importance to the field. The focus on Na(+) in the 2014 symposium stemmed from the multitude of recent studies that point to the importance of maintaining Na(+) homeostasis in the heart, as disruption of homeostatic processes are increasingly identified in cardiac disease states. Understanding how disruption in cardiac Na(+)-based processes leads to derangement in multiple cardiac components at the level of the cell and to then connect these perturbations to emergent behaviour in the heart to cause disease is a critical area of research. The ubiquity of disruption of Na(+) channels and Na(+) homeostasis in cardiac disorders of excitability and mechanics emphasizes the importance of a fundamental understanding of the associated mechanisms and disease processes to ultimately reveal new targets for human therapy.

Delayed afterdepolarizations generate both triggers and a vulnerable substrate promoting reentry in cardiac tissue

BACKGROUND

Delayed afterdepolarizations (DADs) have been well characterized as arrhythmia triggers, but their role in generating a tissue substrate vulnerable to reentry is not well understood.

OBJECTIVE

The purpose of this study was to test the hypothesis that random DADs can self-organize to generate both an arrhythmia trigger and a vulnerable substrate simultaneously in cardiac tissue as a result of gap junction coupling.

METHODS

Computer simulations in 1-dimensional cable and 2-dimensional tissue models were performed. The cellular DAD amplitude was varied by changing the strength of sarcoplasmic reticulum calcium release. Random DAD latency and amplitude in different cells were simulated using gaussian distributions.

RESULTS

Depending on the strength of spontaneous sarcoplasmic reticulum calcium release and other conditions, random DADs in cardiac tissue resulted in the following behaviors: (1) triggered activity (TA); (2) a vulnerable tissue substrate causing unidirectional conduction block and reentry by inactivating sodium channels; (3) both triggers and a vulnerable substrate simultaneously by generating TA in regions next to regions with subthreshold DADs susceptible to unidirectional conduction block and reentry. The probability of the latter 2 behaviors was enhanced by reduced sodium channel availability, reduced gap junction coupling, increased tissue heterogeneity, and less synchronous DAD latency.

CONCLUSION

DADs can self-organize in tissue to generate arrhythmia triggers, a vulnerable tissue substrate, and both simultaneously. Reduced sodium channel availability and gap junction coupling potentiate this mechanism of arrhythmias, which are relevant to a variety of heart disease conditions.

Perspective: a dynamics-based classification of ventricular arrhythmias

Despite key advances in the clinical management of life-threatening ventricular arrhythmias, culminating with the development of implantable cardioverter-defibrillators and catheter ablation techniques, pharmacologic/biologic therapeutics have lagged behind. The fundamental issue is that biological targets are molecular factors. Diseases, however, represent emergent properties at the scale of the organism that result from dynamic interactions between multiple constantly changing molecular factors. For a pharmacologic/biologic therapy to be effective, it must target the dynamic processes that underlie the disease. Here we propose a classification of ventricular arrhythmias that is based on our current understanding of the dynamics occurring at the subcellular, cellular, tissue and organism scales, which cause arrhythmias by simultaneously generating arrhythmia triggers and exacerbating tissue vulnerability. The goal is to create a framework that systematically links these key dynamic factors together with fixed factors (structural and electrophysiological heterogeneity) synergistically promoting electrical dispersion and increased arrhythmia risk to molecular factors that can serve as biological targets. We classify ventricular arrhythmias into three primary dynamic categories related generally to unstable Ca cycling, reduced repolarization, and excess repolarization, respectively. The clinical syndromes, arrhythmia mechanisms, dynamic factors and what is known about their molecular counterparts are discussed. Based on this framework, we propose a computational-experimental strategy for exploring the links between molecular factors, fixed factors and dynamic factors that underlie life-threatening ventricular arrhythmias. The ultimate objective is to facilitate drug development by creating an in silico platform to evaluate and predict comprehensively how molecular interventions affect not only a single targeted arrhythmia, but all primary arrhythmia dynamics categories as well as normal cardiac excitation-contraction coupling.

Mechanisms of ventricular arrhythmias: from molecular fluctuations to electrical turbulence

Ventricular arrhythmias have complex causes and mechanisms. Despite extensive investigation involving many clinical, experimental, and computational studies, effective biological therapeutics are still very limited. In this article, we review our current understanding of the mechanisms of ventricular arrhythmias by summarizing the state of knowledge spanning from the molecular scale to electrical wave behavior at the tissue and organ scales and how the complex nonlinear interactions integrate into the dynamics of arrhythmias in the heart. We discuss the challenges that we face in synthesizing these dynamics to develop safe and effective novel therapeutic approaches.

SKF-96365 strongly inhibits voltage-gated sodium current in rat ventricular myocytes

SKF-96365 (1-(beta-[3-(4-methoxy-phenyl) propoxy]-4-methoxyphenethyl)-1H-imidazole hydrochloride) is a general TRPC channel antagonist commonly used to characterize the potential functions of TRPC channels in cardiovascular system. Recent reports showed that SKF-96365 induced a reduction in cardiac conduction. The present study investigates whether the reduced cardiac conduction caused by SKF-96365 is related to the blockade of voltage-gated sodium current (I Na) in rat ventricular myocytes using the whole-cell patch voltage-clamp technique. It was found that SKF-96365 inhibited I Na in rat ventricular myocytes in a concentration-dependent manner. The compound (1 μM) negatively shifted the potential of I Na availability by 9.5 mV, increased the closed-state inactivation of I Na, and slowed the recovery of I Na from inactivation. The inhibition of cardiac I Na by SKF-96365 was use-dependent and frequency-dependent, and the IC₅₀ was decreased from 1.36 μM at 0.5 Hz to 1.03, 0.81, 0.61, 0.56 μM at 1, 2, 5, 10 Hz, respectively. However, the selective TRPC3 antagonist Pyr3 decreased cardiac I Na by 8.5% at 10 μM with a weak use and frequency dependence. These results demonstrate that the TRPC channel antagonist SKF-96365 strongly blocks cardiac I Na in use-dependent and frequency-dependent manners. Caution should be taken for interpreting the alteration of cardiac electrical activity when SKF-96365 is used in native cells as a TRPC antagonist.

A combination of topical antiseptics for the treatment of sore throat blocks voltage-gated neuronal sodium channels

Amylmetacresol and dichloro-benzylalcohol are ingredients of lozenges used for the treatment of sore throat. In a former in vitro study, a local anaesthetic-like effect of these substances has been described. Since amylmetacresol and dichloro-benzylalcohol are co-administered in over-the-counter lozenges, the intention of this study is to evaluate the in vitro effects of the combination of these compounds on the voltage-gated sodium channel. We analysed the block of inward sodium currents induced by the combination of amylmetacresol, dichloro-benzylalcohol and the local anaesthetic lidocaine. Tonic and use-dependent block and effects on the inactivated channel state of the neuronal sodium channel were examined. Therefore, the α-subunit of the voltage-gated NaV1.2 sodium channel was heterologously expressed in HEK 293 cells in vitro. Inward sodium currents were investigated in the whole-cell configuration of the patch-clamp technique. The combination of amylmetacresol and dichloro-benzylalcohol and the combination of amylmetacresol and lidocaine induced a block of resting and inactivated sodium channels both displaying a pronounced block at the inactivated channel state. In addition, the combination of all three compounds also resulted in a voltage-dependent block of inward sodium currents. While use-dependent block by co-application of amylmetacresol and dichloro-benzylalcohol was moderate (<20 %), lidocaine and amylmetacresol induced a robust use-dependent block (up to 50 %). This study demonstrates local anaesthetic-like effects of a combination of amylmetacresol and dichloro-benzylalcohol as established ingredients of lozenges. In the presence of amylmetacresol, dichloro-benzylalcohol and lidocaine, a prominent block of inward sodium currents is apparent.

Electrophysiological effects of ivabradine in dog and human cardiac preparations: potential antiarrhythmic actions

Ivabradine is a novel antianginal agent which inhibits the pacemaker current. The effects of ivabradine on maximum rate of depolarization (V(max)), repolarization and spontaneous depolarization have not yet been reported in human isolated cardiac preparations. The same applies to large animals close to human in heart size and spontaneous frequency. Using microelectrode technique action potential characteristics and by applying patch-clamp technique ionic currents were studied. Ivabradine exerted concentration-dependent (0.1-10 μM) decrease in the amplitude of spontaneous diastolic depolarization and reduction in spontaneous rate of firing of action potentials and produced a concentration- and frequency-dependent V(max) block in dog Purkinje fibers while action potential duration measured at 50% of repolarization was shortened. In the presence of ivabradine, at 400 ms cycle length, V(max) block developed with an onset kinetic rate constant of 13.9 ± 3.2 beat(-1) in dog ventricular muscle. In addition to a fast recovery of V(max) from inactivation (τ=41-46 ms) observed in control, a second slow component for recovery of V(max) was expressed (offset kinetics of V(max) block) having a time constant of 8.76 ± 1.34 s. In dog after attenuation of the repolarization reserve ivabradine moderately but significantly lengthened the repolarization. In human, significant prolongation of repolarization was only observed at 10 μM ivabradine. Ivabradine in addition to the Class V antiarrhythmic effect also has Class I/C and Class III antiarrhythmic properties, which can be advantageous in the treatment of patients with ischemic heart disease liable to disturbances of cardiac rhythm.

Class I/B antiarrhythmic property of ranolazine, a novel antianginal agent, in dog and human cardiac preparations

The aim of this study was to investigate the cellular electrophysiological effects of ranolazine on action potential characteristics. The experiments were carried out in dog and human cardiac preparations using the conventional microelectrode technique. In dog Purkinje fibres ranolazine produced a concentration- and frequency-dependent depression of the maximum rate of depolarization (V(max)) while action potential duration (APD) was shortened. In dog and human right ventricular papillary muscle ranolazine exerted no significant effect on APD, while it produced, like mexiletine, use-dependent depression of V(max) with relatively fast onset and offset kinetics. In dog midmyocardial preparations the drug did not exert statistically significant effect on repolarization at 10 μM, although a tendency toward prolongation was observed at 20 μM. A moderate lengthening of APD(90) by ranolazine was noticed in canine atrial preparations obtained from dogs in sinus rhythm and in tachypacing induced remodelled preparations. Use-dependent depression of V(max) was more pronounced in atria from dogs in sinus rhythm than those in remodelled atria or in the ventricle. These findings indicate that ranolazine, in addition to its known late sodium current blocking effect, also depresses peak I(Na) with class I/B antiarrhythmic characteristics. Although peak I(Na) inhibition by ranolazine is stronger in the atria, it is also substantial (at fast stimulation frequencies) in ventricular preparations. Ranolazine also decreased the dispersion of ventricular repolarization (the difference in APD(90) values between Purkinje fibres and papillary muscles), which can contribute to the antiarrhythmic property of the drug.

Alternans and arrhythmias: from cell to heart

The goal of systems biology is to relate events at the molecular level to more integrated scales from organelle to cell, tissue, and living organism. Here, we review how normal and abnormal excitation-contraction coupling properties emerge from the protein scale, where behaviors are dominated by randomness, to the cell and tissue scales, where heart has to beat with reliable regularity for a lifetime. Beginning with the fundamental unit of excitation-contraction coupling, the couplon where L-type Ca channels in the sarcolemmal membrane adjoin ryanodine receptors in the sarcoplasmic reticulum membrane, we show how a network of couplons with 3 basic properties (random activation, refractoriness, and recruitment) produces the classic physiological properties of excitation-contraction coupling and, under pathophysiological conditions, leads to Ca alternans and Ca waves. Moving to the tissue scale, we discuss how cellular Ca alternans and Ca waves promote both reentrant and focal arrhythmias in the heart. Throughout, we emphasize the qualitatively novel properties that emerge at each new scale of integration.

Topical antiseptics for the treatment of sore throat block voltage-gated neuronal sodium channels in a local anaesthetic-like manner

Lozenges for the treatment of sore throat provide relief of discomfort in cases of oral inflammation. This effect has not been fully explained so far. Here, we have examined the proposition that key components of pharmaceutical preparations for the treatment of sore throat which are routinely regarded antiseptics might have sodium channel-blocking, i.e. local anaesthetic-like effects. We investigated the effects of hexylresorcinol, amylmetacresol and dichloro-benzylalcohol on voltage-operated neuronal (Na(V)1.2) sodium channels heterologously expressed in HEK 293 cells in vitro. Hexylresorcinol, amylmetacresol and dichloro-benzylalcohol reversibly blocked depolarisation-induced whole-cell sodium inward currents. The half-maximum blocking concentrations (EC(50)) at -150 mV were 23.1, 53.6 and 661.6 microM, respectively. Block induced by hexylresorcinol and amylmetacresol was increased at depolarised potentials and use-dependent during trains of depolarisations applied at high frequency (100 Hz) indicating that both drugs bind more tightly to inactivated conformations of the channel. Estimates for the inactivated state affinity were 1.88 and 35 microM for hexylresorcinol and amylmetacresol, respectively. Hexylresorcinol and amylmetacresol are 10-20 fold more potent than the local anaesthetic lidocaine in blocking sodium inward current. Both drugs show an increased effect at depolarised membrane potentials or in conditions of high-frequency discharges.

Regulation of the ankyrin-B-based targeting pathway following myocardial infarction

AIMS

Ion channel reorganization is a critical step in the pro-arrhythmogenic remodelling process that occurs in heart disease. Ankyrin-B (AnkB) is required for targeting and stabilizing ion channels, exchangers, and pumps. Despite a wealth of knowledge implicating the importance of AnkB in human cardiovascular physiology, nothing is known regarding the role of AnkB in common forms of acquired human disease.

METHODS AND RESULTS

We present the first report of AnkB regulation following myocardial infarction (MI). AnkB protein levels were reduced in the infarct border zone 5 days following coronary artery occlusion in the canine. We also observed a dramatic increase in AnkB mRNA levels 5 days post-occlusion. Surprisingly, the expression of the upstream AnkB cytoskeletal component beta2-spectrin was unchanged in post-infarct tissues. However, protein levels and/or membrane expression of downstream AnkB-associated ion channels and transporters Na+/K+ ATPase, Na+/Ca2+ exchanger, and IP3 receptor were altered 5 days post-occlusion. Interestingly, protein levels of the protein phosphatase 2A, an AnkB-associated signalling protein, were significantly affected 5 days post-occlusion. AnkB and PP2A protein levels recovered by 14 days post-occlusion, whereas Na+/K+ ATPase levels recovered by 2 months post-occlusion.

CONCLUSION

These findings reveal the first evidence of ankyrin remodelling following MI and suggest an unexpected divergence point for regulation between ankyrin and the underlying cytoskeletal network. These findings suggest a logical, but unexpected, molecular mechanism underlying ion channel and transporter remodelling following MI.

Omega-3 polyunsaturated fatty acids inhibit transient outward and ultra-rapid delayed rectifier K+currents and Na+current in human atrial myocytes

AIMS

The omega-3 (n-3) polyunsaturated fatty acids (omega-3 PUFAs) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from fish oil were recently reported to have an anti-atrial fibrillation effect in humans; however, the ionic mechanisms of this effect are not fully understood. The present study was designed to determine the effects of EPA and DHA on transient outward and ultra-rapid delayed rectifier potassium currents (I(to) and I(Kur)) and the voltage-gated sodium current (I(Na)) in human atrial myocytes.

METHODS AND RESULTS

A whole-cell patch voltage clamp technique was employed to record I(to) and I(Kur), and I(Na) in human atrial myocytes. It was found that EPA and DHA inhibited I(to) in a concentration-dependent manner (IC(50): 6.2 microM for EPA; 4.1 microM for DHA) and positively shifted voltage-dependent activation of the current. In addition, I(Kur) was suppressed by 1-50 microM EPA (IC(50): 17.5 microM) and DHA (IC(50): 4.3 microM). Moreover, EPA and DHA reduced I(Na) in human atrial myocytes in a concentration-dependent manner (IC(50): 10.8 microM for EPA; 41.2 microM for DHA) and negatively shifted the potential of I(Na) availability. The I(Na) block by EPA or DHA was use-independent.

CONCLUSION

The present study demonstrates for the first time that EPA and DHA inhibit human atrial I(to), I(Kur), and I(Na) in a concentration-dependent manner; these effects may contribute, at least in part, to the anti-atrial fibrillation of omega-3 PUFAs in humans.

Potential role of isoketals formed via the isoprostane pathway of lipid peroxidation in ischemic arrhythmias

Unabated reactive oxygen species (ROS) are potentiated by an ischemia-induced shift in anaerobic metabolism, which generates superoxide anion upon reperfusion and reintroduction of oxygen. ROS can modify protein structure and function in fundamental ways, one of which is by forming reactive lipid species from the oxidation of lipids. In this review, we discuss these pathways and discuss the literature that shows that these species can produce dramatic effects on cardiac ion channel function (eg, Na+ channel function). Furthermore, we review what is known about the generation of such in the highly remodeled post myocardial infarction substrate. We suggest prevention of adduction of these highly reactive compounds would be antiarrhythmic.

Antiarrhythmic effect of newly synthesized compound 44Bu on model of aconitine-induced arrhythmia — Compared to lidocaine

The antiarrhythmic action of the newly developed compound 44Bu (an original compound that was synthesized at our Faculty of Pharmacy) was tested on a model of aconitine-induced arrhythmia and compared with the effect of lidocaine. Both tested substances were administered either as therapeutic or prophylactic agents. 44Bu was highly effective in reducing the occurrence of ventricular fibrillation from 94% to 8% by therapeutic administration, and to 0% by prophylactic administration. The overall mortality rate was significantly reduced by 44Bu from 100% to 25% in the case of therapeutic administration, and to 0% in the case of prophylactic administration. In contrast, there was not any significant difference between therapeutic and prophylactic administration of lidocaine. The occurrence of ventricular fibrillation dropped from 94% to 50% with therapeutic administration, and to 67% with prophylactic administration of lidocaine. The overall mortality rate was significantly reduced from 100% to 63% and to 67%, respectively. We conclude that the 44Bu compound is a highly effective agent in suppressing aconitine-induced arrhythmias. The antiarrhythmic effect of 44Bu was significantly more evident in comparison with lidocaine, particularly in the case of its prophylactic administration.

Regulation of voltage-gated cardiac sodium current by epidermal growth factor receptor kinase in guinea pig ventricular myocytes

Voltage-gated cardiac fast sodium channel current (I(Na)) plays a critical role in the initiation and propagation of the myocardial action potential, and regulation of cardiac I(Na) by protein tyrosine kinases (PTKs) is not well documented, though it is known that ion channels are among the targets of PTKs. The present study was therefore designed to investigate whether/how cardiac I(Na) was modulated by PTKs in guinea pig ventricular myocytes using whole-cell patch clamp and immunoprecipitation and Western blotting approaches. It was found that cardiac I(Na) was enhanced by epidermal growth factor (EGF), and the effect was antagonized by the selective epidermal growth factor receptor (EGFR) kinase inhibitor tyrphostin AG556 while potentiated by orthovanadate (a protein tyrosine phosphatase (PTP) inhibitor). In addition, AG556 inhibited, while orthovanadate increased I(Na), and the inhibition of I(Na) by AG556 was antagonized by orthovanadate. Immunoprecipitation and Western blotting analysis demonstrated that tyrosine phosphorylation level of cardiac sodium channels was enhanced by EGF or orthovanadate, and reduced by AG556. The AG556-induced reduction of phosphorylation level was significantly reversed by orthovanadate. Our results demonstrate the novel information that EGFR kinase enhances, and PTPs reduce native cardiac I(Na) in guinea pig ventricular myocytes.

Arrhythmogenic Ion-Channel Remodeling in the Heart: Heart Failure, Myocardial Infarction, and Atrial Fibrillation

Rhythmic and effective cardiac contraction depends on appropriately timed generation and spread of cardiac electrical activity. The basic cellular unit of such activity is the action potential, which is shaped by specialized proteins (channels and transporters) that control the movement of ions across cardiac cell membranes in a highly regulated fashion. Cardiac disease modifies the operation of ion channels and transporters in a way that promotes the occurrence of cardiac rhythm disturbances, a process called “arrhythmogenic remodeling.” Arrhythmogenic remodeling involves alterations in ion channel and transporter expression, regulation and association with important protein partners, and has important pathophysiological implications that contribute in major ways to cardiac morbidity and mortality. We review the changes in ion channel and transporter properties associated with three important clinical and experimental paradigms: congestive heart failure, myocardial infarction, and atrial fibrillation. We pay particular attention to K+, Na+, and Ca2+channels; Ca2+transporters; connexins; and hyperpolarization-activated nonselective cation channels and discuss the mechanisms through which changes in ion handling processes lead to cardiac arrhythmias. We highlight areas of future investigation, as well as important opportunities for improved therapeutic approaches that are being opened by an improved understanding of the mechanisms of arrhythmogenic remodeling.

Sodium Channel Variants in Heart Disease: Expanding Horizons

Inherited arrhythmia syndromes have advanced our understanding of cardiac sodium (Na) channel function in health and disease. Long QT syndrome (LQT3) is consistently caused by increased net Na current secondary to inactivation defects, which give rise to persistent Na current. Conversely, various gating changes that ultimately result in reduced Na current may elicit Brugada syndrome, conduction disease, atrial standstill, and sinus node disease. Emerging insights now also link these gating defects to enhanced arrhythmia susceptibility in common, acquired, disease. For instance, action potential prolongation in congestive heart failure may be explained by increased persistent Na current. Of note, recent studies have also linked Na current reduction to structural cardiac defects, notably cardiac fibrosis, dilated cardiomyopathy and, possibly, arrhythmogenic right ventricular cardiomyopathy. These structural changes may also be conducive to (reentrant) arrhythmias. Clearly, these observations highlight the cardiac Na channel as an interesting target for novel therapy strategies.

Sodium current function in adult and aged canine atrial cells

The incidence of atrial fibrillation increases with age, but it is unknown whether there are changes in the intrinsic function of Na+ currents in cells of the aged atria. Thus, we studied right (RA) and left (LA) atrial cells from two groups of dogs, adult and aged (>8 yr), to determine the change in Na+ currents with age. In this study all dogs were in normal sinus rhythm. Whole cell voltage clamp techniques were used to compare the Na+ currents in the two cell groups. Immunocytochemical studies were completed for the Na+ channel protein Na(v)1.5 to determine whether there was structural remodeling of this protein with age. In cells from aged animals, we found that Na+ currents are similar to those we measured in adult atria. However, Na+ current (I(Na)) density of the aged atria differed depending on the atrial chamber with LA cell currents being larger than RA cell currents. Thus with age, the difference in I(Na) density between atrial chambers remains. I(Na) kinetic differences between aged and adult cells included a significant acceleration into the inactivated state and an enhanced use-dependent decrease in peak current in aged RA cells. Finally, there is no structural remodeling of the cardiac Na+ channel protein Na(v)1.5 in the aged atrial cell. In conclusion, with age there is no change in I(Na) density, but there are subtle kinetic differences contributing to slight enhancement of use dependence. There is no structural remodeling of the fast Na+ current protein with age.

Remodeling in cells from different regions of the reentrant circuit during ventricular tachycardia

BACKGROUND

Anisotropic reentrant excitation occurs in the remodeled substrate of the epicardial border zone (EBZ) of the 5-day infarcted canine heart. Reentry is stabilized because of the formation of functional lines of block. We hypothesized that regional differences of ionic currents in cells of the EBZ form these lines of block. Therefore, we first mapped reentrant circuits of sustained tachycardias, then dispersed cells (infarct zone cells, IZs) from the central common pathway of the circuit (IZc) as well as from the other side of the line of block (outer pathway, IZo) for study.

METHODS AND RESULTS

We mapped reentrant circuits in the EBZ of infarcted hearts during sustained ventricular tachycardias (>30 seconds, n=17 episodes, cycle lengths=218+/-7.9 ms). INa density was reduced in both IZc and IZo, and the kinetic properties of IZc INa were markedly altered versus IZo. Structural remodeling of the sodium channel protein Nav1.5 occurred in IZs, with cell surface localization differing from normal cells. Both IZc and IZo have similar but reduced ICaL, whereas IZc showed changes in Ca2+ current kinetics with an acceleration of current decay. Computer simulations of the 2D EBZ showed that incorporating only differences between INa in IZc and IZo prevented stability of the reentrant circuit. Incorporating only differences between ICaL in the IZc and IZo cells also prevented stability of the circuit. However, incorporating both INa and ICaL current differences stabilized the simulated reentrant circuit, and lines of block formed between the 2 distinct regions.

CONCLUSIONS

Despite differences in INa and ICaL properties in cells of the center and outer pathways of a reentrant circuit, the resulting changes in effective refractory periods tend to stabilize reentry in this remodeled substrate.

Effects of Na(+) and K(+) channel blockade on vulnerability to and termination of fibrillation in simulated normal cardiac tissue

Na(+) and K(+) channel-blocking drugs have anti- and proarrhythmic effects. Their effects during fibrillation, however, remain poorly understood. We used computer simulation of a two-dimensional (2-D) structurally normal tissue model with phase I of the Luo-Rudy action potential model to study the effects of Na(+) and K(+) channel blockade on vulnerability to and termination of reentry in simulated multiple-wavelet and mother rotor fibrillation. Our main findings are as follows: 1) Na(+) channel blockade decreased, whereas K(+) channel blockade increased, the vulnerable window of reentry in heterogeneous 2-D tissue because of opposing effects on dynamical wave instability. 2) Na(+) channel blockade increased the cycle length of reentry more than it increased refractoriness. In multiple-wavelet fibrillation, Na(+) channel blockade first increased and then decreased the average duration or transient time (<T(s)>) of fibrillation. In mother rotor fibrillation, Na(+) channel blockade caused peripheral fibrillatory conduction block to resolve and the mother rotor to drift, leading to self-termination or sustained tachycardia. 3) K(+) channel blockade increased dynamical instability by steepening action potential duration restitution. In multiple-wavelet fibrillation, this effect shortened <T(s)> because of enhanced wave instability. In mother rotor fibrillation, this effect converted mother rotor fibrillation to multiple-wavelet fibrillation, which then could self-terminate. Our findings help illuminate, from a theoretical perspective, the possible underlying mechanisms of termination of different types of fibrillation by antiarrhythmic drugs.

Effects of Na+ channel and cell coupling abnormalities on vulnerability to reentry: a simulation study

The role of dynamic instabilities in the initiation of reentry in diseased (remodeled) hearts remains poorly explored. Using computer simulations, we studied the effects of altered Na+ channel and cell coupling properties on the vulnerable window (VW) for reentry in simulated two-dimensional cardiac tissue with and without dynamic instabilities. We related the VW for reentry to effects on conduction velocity, action potential duration (APD), effective refractory period dispersion and restitution, and concordant and discordant APD alternans. We found the following: 1) reduced Na+ current density and slowed recovery promoted postrepolarization refractoriness and enhanced concordant and discordant APD alternans, which increased the VW for reentry; 2) uniformly reduced cell coupling had little effect on cellular electrophysiological properties and the VW for reentry. However, randomly reduced cell coupling combined with decoupling promoted APD dispersion and alternans, which also increased the VW for reentry; 3) the combination of decreased Na+ channel conductance, slowed Na+ channel recovery, and cellular uncoupling synergistically increased the VW for reentry; and 4) the VW for reentry was greater when APD restitution slope was steep than when it was flat. In summary, altered Na+ channel and cellular coupling properties increase vulnerability to reentrant arrhythmias. In remodeled hearts with altered Na+ channel properties and cellular uncoupling, dynamic instabilities arising from electrical restitution exert important influences on the VW for reentry.

Electrical remodeling of the epicardial border zone in the canine infarcted heart: a computational analysis

The density and kinetics of several ionic currents of cells isolated from the epicardial border zone of the infarcted heart (IZs) are markedly different from cells from the noninfarcted canine epicardium (NZs). To understand how these changes in channel function affect the action potential of the IZ cell as well as its response to antiarrhythmic agents, we developed a new ionic model of the action potential of a cell that survives in the infarct (IZ) and one of a normal epicardial cell (NZ) using formulations based on experimental measurements. The difference in action potential duration (APD) between NZ and IZ cells during steady-state stimulation (basic cycle length = 250 ms) was 6 ms (156 ms in NZ and 162 ms in IZ). However, because IZs exhibit postrepolarization refractoriness, the difference in the effective refractory period (ERP), calculated using a propagation model of a single fiber of 100 cells, was 43 ms (156 ms in NZ and 199 ms in IZ). Either an increase in L-type Ca(2+) current (to simulate the effects of BAY Y5959) or a decrease of both or either delayed rectifier currents (e.g., to simulate the effects of azimilide, sotalol, and chromanol) had significant effects on NZ ERP. In contrast, the effects of these agents in IZs were minor, in agreement with measurements in the in situ canine infarcted heart. Therefore 1) because IZs exhibit postrepolarization refractoriness, conclusions drawn from APD measurements cannot be extrapolated directly to ERPs; 2) ionic currents that are the major determinants of APD and the ERP in NZs are less important in IZs; and 3) differential effects of either BAY Y5959 or azimilide in NZs versus IZs are predicted to decrease ERP dispersion and in so doing prevent initiation of arrhythmias in a substrate of inhomogeneous APD/ERPs.

Interaction between fast and ultra-slow inactivation in the voltage-gated sodium channel. Does the inactivation gate stabilize the channel structure?

Recently, we reported that mutation A1529D in the domain (D) IV P-loop of the rat skeletal muscle Na(+) channel mu(1) (DIV-A1529D) enhanced entry to an inactivated state from which the channels recovered with an abnormally slow time constant on the order of approximately 100 s. Transition to this "ultra-slow" inactivated state (USI) was substantially reduced by binding to the outer pore of a mutant mu-conotoxin GIIIA. This indicated that USI reflected a structural rearrangement of the outer channel vestibule and that binding to the pore of a peptide could stabilize the pore structure (Hilber, K., Sandtner, W., Kudlacek, O., Glaaser, I. W., Weisz, E., Kyle, J. W., French, R. J., Fozzard, H. A., Dudley, S. C., and Todt, H. (2001) J. Biol. Chem. 276, 27831-27839). Here, we tested the hypothesis that occlusion of the inner vestibule of the Na(+) channel by the fast inactivation gate inhibits ultra-slow inactivation. Stabilization of the fast inactivated state (FI) by coexpression of the rat brain beta(1) subunit in Xenopus oocytes significantly prolonged the time course of entry to the USI. A reduction in USI was also observed when the FI was stabilized in the absence of the beta(1) subunit, suggesting a causal relation between the occurrence of the FI and inhibition of USI. This finding was further confirmed in experiments where the FI was destabilized by introducing the mutations I1303Q/F1304Q/M1305Q. In DIV-A1529D + I1303Q/F1304Q/M1305Q channels, occurrence of USI was enhanced at strongly depolarized potentials and could not be prevented by coexpression of the beta(1) subunit. These results strongly suggest that FI inhibits USI in DIV-A1529D channels. Binding to the inner pore of the fast inactivation gate may stabilize the channel structure and thereby prevent USI. Some of the data have been published previously in abstract form (Hilber, K., Sandtner, W., Kudlacek, O., Singer, E., and Todt, H. (2002) Soc. Neurosci. Abstr. 27, program number 46.12).

The selectivity filter of the voltage-gated sodium channel is involved in channel activation

Amino acids located in the outer vestibule of the voltage-gated Na+ channel determine the permeation properties of the channel. Recently, residues lining the outer pore have also been implicated in channel gating. The domain (D) IV P-loop residue alanine 1529 forms a part of the putative selectivity filter of the adult rat skeletal muscle (mu1) Na+ channel. Here we report that replacement of alanine 1529 by aspartic acid enhances entry to an ultra-slow inactivated state. Ultra-slow inactivation is characterized by recovery time constants on the order of approximately 100 s from prolonged depolarizations and by the fact that entry to this state can be reduced by binding to the pore of a mutant mu-conotoxin GIIIA, suggesting that ultra-slow inactivation may reflect a structural rearrangement of the outer vestibule. The voltage dependence of ultra-slow inactivation in DIV-A1529D is U-shaped, with a local maximum near -60 mV, whereas activation is maximal only above -20 mV. Furthermore, a train of brief depolarizations produces more ultra-slow inactivation than a single maintained depolarization of the same duration. These data suggest that ultra-slow inactivation emanates from "partially activated" closed states and that the P-loop in DIV may undergo a conformational change during channel activation, which is accentuated by DIV-A1529D.

The cardiac sodium channel: gating function and molecular pharmacology

Cardiac sodium (Na) channels are dynamic molecules that undergo rapid structural changes in response to the changing electrical field in the myocardium. Inherited mutations in SCN5A, the gene encoding the cardiac Na channel, provoke life-threatening cardiac arrhythmias, often by modifying these voltage-dependent conformational changes. These disorders (i.e. the long QT syndrome and Brugada syndrome) may serve as valuable models for understanding the mechanistic linkages between Na channel dysfunction and cardiac arrhythmias in more common, acquired conditions such as cardiac ischemia. In addition, the balance between therapeutic and adverse effects from Na channel blockade by antiarrhythmic compounds may be shifted by subtle alterations in Na channel function. This review examines recent studies that tie key loci in the Na channel primary sequence to its dynamic function, while examining the emerging themes linking Na channel structure, function, and pharmacology to inherited and acquired disorders of cardiac excitability.

Phenol derivatives accelerate inactivation kinetics in one inactivation-deficient mutant human skeletal muscle Na(+) channel

Altered inactivation kinetics in skeletal muscle Na(+) channels due to mutations in the encoding gene are causal for the alterations in muscle excitability in nondystrophic myotonia. Na(+) channel blockers like lidocaine and mexiletine, suggested for therapy of myotonia, do not reconstitute inactivation in channels with defective inactivation in vitro. We examined the effects of four methylated and/or halogenated phenol derivatives on one heterologously expressed inactivation-deficient Paramyotonia congenita-mutant (R1448H) muscle Na(+) channel in vitro. All these compounds accelerated delayed inactivation of R1448H-whole-cell currents during a depolarization and delayed accelerated recovery from inactivation. The potency of these effects paralleled the potency of the drugs to block the peak current amplitude. We conclude that the investigated phenol derivatives affect inactivation-deficient Na(+) channels more specifically than lidocaine and mexiletine. However, for all compounds, the effect on inactivation was accompanied by a substantial block of the peak current amplitude.

Late post-myocardial infarction induces a tetrodotoxin-resistant Na(+)Current in rat cardiomyocytes

Left ventricular remodeling after myocardial infarction is accompanied by electrical abnormalities that might predispose to rhythm disturbances. To get insight into the ionic mechanisms involved, we studied myocytes isolated from four different regions of the rat ventricles, 4-6 months after ligation of the left coronary artery. Using the whole-cell patch-clamp technique, we never observed T-type Ca(2+)current in both diseased and control hearts. In contrast, in 41 out of 78 cells isolated from 16 post-myocardial infarcted rats, analysed in the presence of 30 m m Na(+)ions, we found a tetrodotoxin (TTX)-resistant Na(+)current with quite variable amplitude in every investigated region. Albeit being resistant to 100 microM TTX, this Na(+)-dependent current was highly sensitive to lidocaine since 3 microM lidocaine induced about 65% tonic block. It was also inhibited by 5 microM nifedipine and 2 m m Co(2+), but was insensitive to 100 microM Ni(2+). The TTX-resistant Na(+)channel availability was shifted rightward by 25-30 mV with respect to TTX-sensitive Na(+)current; therefore, a large "window current" might flow in the voltage range from -70 to -20 mV. In conclusion, in late post-myocardial infarction, a Na(+)current with specific kinetics and pharmacology may provide inward charges in a critical range of membrane voltages that are able to alter action potential time course and trigger ventricular arrhythmia. These apparent new characteristics of the Na(+)channel might result in part from environmental changes during heart remodeling.

Myocardial perfusion and wall motion in infarction border zone: assessment by myocardial contrast echocardiography

Several mechanisms have been proposed to explain the decreased wall motion (WM) at the borders of myocardial infarction (MI). We used myocardial contrast echocardiography (MCE) to investigate the relation of perfusion to WM in infarcted border zones (BZs) 6 weeks after MI in 5 sheep. After quantifying the extent of WM abnormality and the perfusion defect, normal (NL), infarcted, and BZs were defined. Peak intensity after contrast was measured in acoustic units (AU). Radiolabeled microspheres were injected to measure regional blood flow. The heart was stained with 2,3, 5-triphenyltetrazolium chloride (TTC). The perfusion defect on MCE was 33% +/- 7% of the total myocardial area and correlated well with TTC (r = 0.92, P <.03). The BZ was 8% +/- 5% of the total myocardial area. Peak intensity after contrast was decreased in MI compared with BZ and NL (MI: 2.5 +/- 1.9 AU, BZ: 8.0 +/- 3.8 AU, P <.005; NL: 10.2 +/- 6.9 AU, P <.02) and comparable in NL and BZ. The blood flow measured by microspheres was not different in NL and BZ but was decreased in MI (NL: 1.6 mL/g/min, BZ: 1.5 +/- 0.5 mL/g/min, MI: 0.7 +/- 0.5 mL/g/min; P <.0001). In this model of chronic ovine MI, the BZ was small and its perfusion was preserved. These findings support the hypothesis that tethering of normal myocardial segments explains the abnormal wall motion noted at the borders of MI.